WO 2006/118974 discloses aqueous binder latices which are particularly suitable as binders in water-borne base coats useful in the production of base coat/clear coat two-layer coatings. The aqueous binder latices are produced by multistage emulsion polymerization; olefinically polyunsaturated monomers are copolymerized in all the stages of the emulsion polymerization and olefinically monounsaturated monomers with acid groups are copolymerized in the first stage of the emulsion polymerization. After completion of the first stage of the emulsion polymerization, the acid groups are neutralized.
Surprisingly, the color purity of pigmented coating compositions formulated with the aqueous binder latices disclosed in WO 2006/118974 and of coatings applied from said coating compositions can be improved, if the monomer composition of the monomer mixtures employed during the preparation of those aqueous binder latices is selected in such a manner that the calculated glass transition temperature (Tg) of a copolymer composed of a combination of only the olefinically monounsaturated monomers of the monomer mixtures is in the range from 30 to 100° C. For example, in case of non-chromatic color shades such as white or silver-metallic color shades improved color purity means that those color shades show less undertone. The effect of showing less undertone can be perceived by the human eye and can also be determined colorimetrically; the colors are closer to the L* axis, i.e. the absolute values of a* and/or b* are smaller (brightness L*, red-green value a* and yellow-blue value b* according to the CIELab system, see DIN 6174). In case of black solid colors (black single-tone color shades) as well as black effect colors (black color shades having a pigment content comprising at least one special effect-imparting pigment) improved color purity means that those color shades show less yellowness or a higher so-called jetness (degree of blackness). Furthermore, in case of colored shades, in particular colored effect shades, such as colored effect shades having pigment contents comprising special effect pigments (metal pigments, mica pigments, etc.) improved color purity means a higher saturation of chroma C* (chroma C* according to the CIELab system, see DIN 6174), which can also be perceived by the human eye.
Color capability can be improved with the aqueous binder latices according to the invention. In other words, if the aqueous binder latices according to the invention are used, a better access becomes available to color shades that were difficult or even impossible to achieve when formulating aqueous coating compositions with the aqueous binder latices disclosed in WO 2006/118974.